What is a power transformer?
A power transformer is a static electrical device that transfers energy between two or more circuits by electromagnetic induction. It steps voltage up or down so electricity can be transmitted and distributed efficiently. Unlike a rotating machine, it has no moving parts in the energy path: alternating current in one winding sets up a magnetic flux in a laminated steel core, and that flux induces a voltage in a second winding. The ratio of the two voltages follows the ratio of the turns.
In the transmission and distribution chain, power transformers are the equipment that makes long-distance transmission practical. Generating stations produce at a few kilovolts to roughly 25 kV, and by the time power reaches a city it may have passed through four or five transformers.
Working principle
Faraday’s law in practice
The core carries a time-varying flux. Winding turns linked by that flux see an induced electromotive force, so the primary-to-secondary voltage ratio equals the turns ratio (V1/V2 = N1/N2). Power is conserved apart from losses, so current transforms inversely.
Why the core is laminated
Changing flux also induces unwanted circulating currents in the core itself. Laminating the core into thin, insulated, grain-oriented silicon steel sheets cuts those eddy currents and reduces no-load loss. Modern cores use step-lap or mitred joints to keep the reluctance of the magnetic circuit low.
Where the losses go
- No-load (core) loss — present whenever the unit is energised, regardless of load. It is set by core material and flux density.
- Load (copper) loss — varies with the square of the load current, and is set by conductor cross-section and winding design.
The split matters commercially: a transformer that runs lightly loaded for years is dominated by no-load loss, while a heavily loaded unit is dominated by load loss. Read more in our guide to losses and total cost of ownership.
Types of power transformers
- Large power transformers, for utility and grid applications
- High voltage transformers, 110 kV and above for transmission
- Generator transformers, GSU units for power plants
- Oil-immersed transformers, the standard choice for outdoor and high-rating duty
- Dry-type transformers, for indoor installation where oil is not acceptable
- Compact substations, factory-assembled units for distribution networks
By cooling and insulation
Oil-immersed units use mineral or ester oil as both insulation and coolant; dry-type units use cast resin or air. Cooling class follows rating, from ONAN through ONAF to OFAF and OFWF. Our cooling methods guide sets out how each class is chosen.
Key technical parameters
| Parameter | What it means | Typical range |
|---|---|---|
| Rated power | Apparent power the unit can deliver continuously at rated voltage | Up to several hundred MVA |
| Voltage ratio | Primary to secondary rated voltages | Set by the grid interface |
| Impedance voltage | Short-circuit impedance, as a percentage | Typically 10% to 18% |
| No-load loss | Core loss at rated voltage | Specified per project |
| Load loss | Winding loss at rated current | Specified per project |
| Vector group | Phase displacement between windings | e.g. YNd11, Dyn11 |
| Cooling class | How heat is removed | ONAN / ONAF / OFAF |
Impedance and losses are specified rather than freely chosen: they set voltage regulation and short-circuit withstand on one side, and operating cost on the other. For sizing guidance see our guide on calculating transformer capacity.
Where power transformers are used
- Generation — GSU transformers at power plants, plus station service transformers
- Transmission — substations at 110 kV and above, linking voltage levels
- Distribution — substations stepping down to medium and low voltage
- Renewable energy — solar and wind collector and step-up duty
- Industry — furnace, rectifier and process supply transformers
Selection checklist for buyers
- Confirm the load: present demand, diversity and a 10 to 20 year growth allowance.
- Fix the voltage ratio against the grid interface, including tap range and tolerance.
- Decide oil-immersed or dry-type from the installation site and fire rules.
- Set impedance with the system study, not by habit.
- Specify losses against a total cost of ownership calculation.
- Agree the test schedule and the standards it is carried out to.
- Agree the tap changer type — off-circuit or on-load.
See our testing laboratory for the routine and type tests we run, and our quality control process for how that is managed.
FAQ
What is the difference between a power transformer and a distribution transformer?
They differ mainly in rating, voltage class and duty rather than in principle. Power transformers work at transmission voltage and carry large ratings, often with forced cooling and on-load tap changers. Distribution transformers step medium voltage down to utilisation voltage and are usually smaller, ONAN-cooled and lightly loaded for much of the day.
How long does a power transformer last?
Service life is set largely by insulation ageing, which follows temperature. A unit operated within its temperature limits and maintained properly can run for 30 years or more. Our maintenance guide covers the routine that supports this.
Which standards apply?
IEC 60076 is the international framework, IEEE C57 covers North American practice, and GB/T 1094 applies in China. Which one governs your order is normally set by the utility or by the market you are building for.
Talk to our engineering team
Hebei Youge Electrical Equipment Co., Ltd. designs and builds power transformers to
IEC 60076 and IEEE C57, with routine tests on every unit and a test report supplied with
the equipment. Send us your rating, voltage class and site conditions and we will come back
with a technical proposal.
Contact us to discuss your project, or browse the
power transformer range.